Diabetes and impaired glucose tolerance is a major cause of morbidity and mortality worldwide. Normal glucose homeostasis requires that insulin is properly synthesized and secreted from the beta cell upon periodic increases in blood glucose. Disturbances in beta cell function could result in the loss of glucose-stimulated insulin secretion, a major cause for type II diabetes. Recent studies demonstrated an association between beta cell function, proliferation, and/or survival with an intracellular signaling pathway termed the unfolded protein response (UPR). Upon accumulation of unfolded proteins in the lumen of the endoplasmic reticulum, signal transduction pathways are activated to increase the protein folding capacity and the protein degradative machinery. In addition, protein synthesis is also transiently attenuated. These responses collectively enable cells to tolerate and survive conditions that disrupt normal protein folding and protein secretion processes in the ER. The long-term goal of this proposal is to understand the molecular mechanism of the UPR through structure/function studies of the molecules involved. A class of novel ER trans-membrane receptors including IRE1, PERK, and ATF6 mediate activation of the UPR. Of these, IRE1 and PERK are more closely related and appear to be activated through a similar mechanism. Therefore, our current proposal will focus on the structure and function of these two ER trans-membrane receptors. Under normal conditions, both IRE1 and PERK are kept in an inactive, monomeric form by binding to ER chaperone BiP. Upon receiving stress signals, BiP is released from the receptors, which dimerize to activate downstream signaling events. We will use high resolution X-ray crystallography to determine the structures of the lumenal activation/dimerization domains of IRE1 and PERK as well as their complexes with BiP. In collaboration with Dr. Randy Kaufman at the University of Michigan, we will use molecular biology and biochemistry tools to further probe the structure and function relationship of these proteins. The results we obtained from these studies have the potential for providing novel insights into human disease and may also lead to new therapeutic strategies.